Queue and Deque
- Java course
- Intermediate
- Lesson 91/116
- 5 min
Learn queues with Queue<E> and Deque<E>.
The Queue<E> interface
Queue<E> is a first-in-first-out (FIFO) order. ArrayDeque is the fastest concrete implementation: Queue<String> todo = new ArrayDeque<>();.
offer(e) appends to the tail, poll() removes and returns the head, peek() returns the head without removing it. If the queue is empty, poll and peek return null; remove throws an exception.
Deque: a double-ended queue
Deque<E> (double-ended queue) allows add and remove at both ends. ArrayDeque implements both Queue and Deque. Methods: addFirst(e), addLast(e), removeFirst(), removeLast(), peekFirst(), peekLast().
For example a browser back/forward simulation uses addLast for forward and removeLast for back. When a stack is needed, push/pop also work on a Deque; the old Stack class is replaced by ArrayDeque.
Practice Build a todo queue
Declare Queue<String> todo = new ArrayDeque<>();. Add two tasks with todo.offer("Send email"); and todo.offer("Meeting");. Pull the first task with String first = todo.poll(); and print it with System.out.println(first);. Print the remaining count with System.out.println(todo.size());.
Starter code
import java.util.Queue;
import java.util.ArrayDeque;
public class Main {
public static void main(String[] args) {
// Declare Queue<String>, add two tasks, poll first and print, print remaining count
}
}Sample solution
import java.util.Queue;
import java.util.ArrayDeque;
public class Main {
public static void main(String[] args) {
Queue<String> todo = new ArrayDeque<>();
todo.offer("Send email");
todo.offer("Meeting");
String first = todo.poll();
System.out.println(first);
System.out.println(todo.size());
}
}Expected output
Send email 1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the queue-add call
todo.____("Send email");todo.offer("Send email");Explanation: The offer method appends an element to the queue tail.
2 What does todo.poll() do?
- Removes and returns the head Correct answer
- Only peeks at the head
- Appends an element to the tail
- Empties the queue
Explanation: poll removes and returns the head; returns null if empty.
3 Match the Queue/Deque methods
| Term | Meaning |
|---|---|
offer(e) |
Appends to tail |
poll() |
Removes the head |
peek() |
Shows the head |
addFirst(e) |
Adds to head |